President Obama implored supporters on Monday for donations to help Democrats in the 15 Senate seats he said were "up for grabs" on Nov. 2.

The president painted a picture of an expansive map of competitive Senate races in a fundraising e-mail for the Democratic Senatorial Campaign Committee (DSCC).

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"I'm counting on the DSCC to win the 15 Senate seats still up for grabs over these last 15 days," Obama wrote, asking for more than $900,000 in donations before Thursday to finance a get-out-the-vote "blitz" over the weekend.

Republicans need to pick up a net gain of 10 Senate seats on Election Day in order to win back a majority in the Senate, and make Sen. Mitch McConnellAddison (Mitch) Mitchell McConnellSenate GOP blocks voting rights bill Schumer, McConnell spar as GOP prepares to block voting bill Trump has 'zero desire' to be Speaker, spokesman says MORE (R-Ky.) the majority leader.

The nonpartisan Cook Political Report lists 11 races as "toss up" contests, nine of which are currently controlled by Democrats. Republicans are also seen by Cook as being well-positioned to pick up Democratic seats in Indiana, Arkansas and North Dakota.

Obama said in his appeal to Democrats that the 15-seat playing field he described — and it's not clear which races exactly make up that list — will be "going right down to the wire." 

The president will be lending his support for vulnerable Democrats, too, with several days of travel and fundraising this week. Obama will attend a DSCC fundraiser in suburban Washington DC on Monday night, and travel to Washington on Thursday for an event for Sen. Patty MurrayPatricia (Patty) Lynn MurrayPublic option fades with little outcry from progressives Senate GOP blocks bill to combat gender pay gap OVERNIGHT ENERGY: Biden suspends Arctic oil leases issued under Trump |  Experts warn US needs to better prepare for hurricane season | Progressives set sights on Civilian Climate Corps MORE (D). He'll attend fundraisers for Sen. Barbara BoxerBarbara Levy BoxerBottom line Trump administration halting imports of cotton, tomatoes from Uighur region of China Biden inaugural committee to refund former senator's donation due to foreign agent status MORE (D-Calif.) and Senate Majority Leader Harry ReidHarry Mason ReidBiden fails to break GOP 'fever' Nevada governor signs law making state first presidential primary Infighting grips Nevada Democrats ahead of midterms MORE (D-Nev.) in their respective states on Friday.

"The DSCC has already begun to shift momentum in our favor in critical races — but Republicans only need to win 10 of the 15 seats up for grabs to take the majority," Obama wrote Monday. "But if you keep moving forward in the face of difficulty, we will win this election."